1. Field of the Invention
The present invention relates to an optical disk apparatus, which records and reproduces information by use of laser beam, and to an optical head incorporated in the optical disk apparatus.
2. Prior Art
As a recording machine prevalent currently, a recording machine utilizing a videotape is in popular use. However, a recording machine utilizing an optical disk has been commercially available. The optical disk is more superior to the videotape in random access performance. From viewpoints of a easiness to handle, a repetitive reproduction, a fact that an image deterioration scarcely occurs due to change over time, compactness, or the like, it is considered that the recording machine using the optical disk will widely spread in the future. Furthermore, in addition to the recording, an optical disk apparatus is utilized for various kinds of applications such as an external recording device of a computer, a recording/reproducing apparatus of musical information, or the like. Therefore, it is considered that the optical disk apparatus gains increasingly in importance in the future.
In Japan, targeting at around Year 2003 to 2005, in a television broadcast, digitization of a satellite broadcasting, and digitization of a ground wave broadcasting are likely to be realized. Currently, a broadcast of higher definition moving pictures than a present analog broadcast will be widespread in ordinary homes, and thus it is considered that a demand for digitally recording this high definition moving picture will increase. In order to record this high definition moving picture for about 2 hours without impairing the quality of images, it is necessary that a large capacity storage of 20 to 25 GB in capacity is provided in a disk having a diameter of 12 cm which has the same size as a compact disk or DVD. In other words, it is necessary that the recording density is increased about 4 to 5 times the present DVD.
In order to increase the recording density, it is necessary that the laser beam source is shortened in wavelength, NA (numerical aperture) of the objective lens is increased, and a spot diameter for recording/reproducing information is reduced. At present, the wavelength of the laser beam source of DVD is about 660 nm, NA of the objective lens is about 0.6, and thus the recording capacity of 4.7 GB in one layer on single side is attained. On the other hand, as the laser beam source of short wavelength, a blue-violet semiconductor laser (wavelength 400 nm) is likely to be put into practical use. When this laser beam source is used, if NA is set to 0.85, 25 GB in one layer on single side can be accomplished.
As a prior art of increasing the NA of the objective lens, it is disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-11402. Here, the NA is increased up to maximum 0.85 by use of the objective lenses of two elements. At this time, there is a problem that, the more the NA increases, the more an aberration generated due to a positional shift of an optical system, error of thickness and tilt of a disk substrate, or the like increases. In view of this problem, in the aforementioned prior art, in order to reduce a coma aberration generated due to the disk tilt, thickness of the substrate is thinned down to 0.1 mm. Moreover, as for a spherical aberration generated by error of thickness of the substrate, thickness of the substrate is detected from a difference between the focal shift signals of a surface of the disk and a record surface, and a position of a collimator lens is changed based thereon to compensate the spherical aberration.
In the aforementioned prior art, when the objective lens is offset from a central axis of the collimator lens in tracking operation in a state that the spherical aberration is corrected, the coma aberration occurs in a direction of shifting the objective lens. In the case where the spherical aberration due to the error of the substrate thicknesses is corrected, an amount of the coma aberration by this offset is small. However, in the case where the correction amount is relatively large as when the spherical aberration equivalent to an interlayer interval is corrected in recording and reproducing to and from two-layer disk, the coma aberration by this offset is affected largely, and a spot is deteriorated by the tracking operation, whereby the recording and reproducing of signals are adversely affected.
In order to reduce the influences of the coma aberration due to the offset of the objective lens, it is necessary to adopt a double servo system in which the optical head itself makes the tracking operation in conformity to revolution of the optical disk. However, since the optical head itself is moved in the double servo system, a mechanism system and a control system become complicated, and costs increase. Furthermore, since a movable part is large, this system is unfit for an increase in speed.
The present invention has been made in view of drawbacks in the aforementioned prior art. It is an object of the present invention to provide an optical head, in which a high NA objective lens is incorporated, and which is suitable for increasing a speed at low costs, and to provide an optical disk apparatus incorporating the same.
According to the present invention, there are provided a mechanism which detects and corrects the spherical aberration due to an error of substrate thicknesses or the spherical aberration generated when recording and reproducing to and from a recording layer consisting of a plurality of layers, and further a mechanism which detects and corrects simultaneously the coma aberration also generated by offset of the objective lens by the tracking operation, thereby attaining the aforementioned objects.
According to the present invention, when the spherical aberration is corrected, since the coma aberration generated by offset of the objective lens decreases, the double servo becomes unnecessary. Therefore, it is possible to realize the high density optical head suitable for an increase in speed at low costs.
The optical head, the information reproducing apparatus, and the information recording/reproducing apparatus according to the present invention have the following characteristics.
(1) The optical head comprises: a laser beam source; an objective lens for condensing beam from the laser beam source on an optical disk; and a detector for detecting reflected light from the optical disk, and further comprises: means for generating a first focal shift signal and a first tracking an error signal by use of an inside light flux containing a light flux center of the reflected light out of the reflected light from the optical disk; and means for generating a second focal shift signal and a second tracking shift signal by use of an outside light flux enclosing the inside light flux out of the reflected light from the optical disk.
(2) In the optical head according to (1), a spherical aberration signal is generated based on a difference between the first focal shift signal and the second focal shift signal, and a coma aberration signal is generated based on a difference between the first tracking shift signal and the second tracking shift signal.
(3) The optical head comprises: a laser beam source; an objective lens for condensing beam from the laser beam source on an optical disk; variable focusing means for moving a focal position of an optical spot condensed by the objective lens; tracking means for making the optical spot track a recording track of the optical disk; a light detector for detecting reflected light from the optical disk; and an operation circuit for generating a reproducing signal, a focal shift signal and a tracking shift signal by use of a detection signal of the light detector. The optical head further comprises: a detected light dividing device for dividing a light flux in the vicinity of an optical axis of the reflected light into a plurality of partial light fluxes to make incident onto the light detector, and also dividing the light flux in a peripheral portion of the reflected light into a plurality of partial light fluxes to make incident onto the light detector. In this case, the operation circuit generates: a first focal shift signal and a first tracking shift signal based on the detection signal of the partial light fluxes derived from the light flux in the vicinity of the optical axis; a second focal shift signal and a second tracking shift signal based on the detection signal of the partial light fluxes derived from the light flux of the peripheral portion; a spherical aberration signal according to a difference signal between the first and second focal shift signals, and a focal shift signal according to a sum signal; and a coma aberration signal according to a difference signal between the first and second tracking shift signals, and a tracking shift signal according to a sum signal.
(4) In the optical head according to (3), the detected light dividing device is divided into 8 regions by a circle, a straight line passing a center of the circle and parallel to a direction relevant to a radial direction of the optical disk, and a straight line passing a center of the circle and parallel to a direction relevant to a revolution direction of the optical disk, and the respective regions are diffraction gratings or polarizing diffraction gratings having different diffraction angles and diffraction directions from each other.
(5) In the optical head according to (4), the light detector has a plurality of light-receiving regions provided in one substrate, generates a reproducing signal from a zero-order diffraction light detection signal of the detected light dividing device, and generates a focal shift signal and a tracking shift signal from xc2x1 first-order diffraction light detection signals.
If a light detector having a constitution, in which a plurality of light-receiving regions are formed on one substrate, the reproducing signal is detected by the zero-order diffraction light of the detected light dividing device, and the focal shift signal and tracking shift signal are detected by the xc2x1 first-order diffraction light as described above, is used as a light detector, it becomes possible to relatively reduce noises of the reproducing signal. In addition, the number of optical light-receiving devices requiring an adjustment becomes only one, and thus it becomes possible to suppress manufacturing costs.
(6) The information reproducing apparatus comprises: a laser beam source; an objective lens for condensing beam from the laser beam source on an optical disk; variable focusing means for moving a focal position of an optical spot condensed by the objective lens; tracking means for making the optical spot track a recording track of the optical disk; spherical aberration adding means for adding a variable spherical aberration to light condensed by the objective lens; coma aberration adding means for adding a variable coma aberration to light condensed by the objective lens; a light detector for detecting reflected light from the optical disk; an operation circuit for generating a reproducing signal, a focal shift signal and a tracking shift signal from a detection signal of the light detector; and servo-control means for controlling the variable focusing means, the tracking means, the spherical aberration adding means and the coma aberration adding means. The information reproducing apparatus further comprises: a detected light dividing device for dividing a light flux in the vicinity of an optical axis of the reflected light into a plurality of partial light fluxes to make incident onto the light detector, and also dividing the light flux in a peripheral portion of the reflected light into a plurality of partial light fluxes to make incident onto the light detector. In this case, the operation circuit generates: a first focal shift signal and a first tracking shift signal based on a detection signal of the partial light fluxes derived from the light flux in the vicinity of the optical axis; a second focal shift signal and a second tracking shift signal based on the detection signal of the partial light fluxes derived from the light flux of the peripheral portion; a spherical aberration signal according to a difference signal between the first and second focal shift signals, and a focal shift signal according to a sum signal; and a coma aberration signal according to a difference signal between the first and second tracking shift signals, and a tracking shift signal according to a sum signal. Moreover, the servo-control means controls: the variable focusing means according to the focal shift signal; the tracking means according to the tracking shift signal; the spherical aberration adding means according to the spherical aberration signal; and the coma aberration adding means according to the coma aberration signal.
(7) In the information reproducing apparatus according to (6), the detected light dividing device is divided into 8 regions by a circle, a straight line passing a center of the circle and parallel to a direction relevant to a radial direction of the optical disk, and a straight line passing a center of the circle and parallel to a direction relevant to a revolution direction of the optical disk, and the respective regions are diffraction gratings or polarizing diffraction gratings having different diffraction angles and diffraction directions from each other.
(8) In the information reproducing apparatus according to (7), the light detector has a plurality of light-receiving regions on one substrate, generates a reproducing signal from a zero-order diffraction light detection signal of the detected light dividing device, and generates the focal shift signal and tracking shift signal from the xc2x1 first-order diffraction light detection signal.
(9) The information recording/reproducing apparatus comprises: a laser beam source; an objective lens for condensing beam from the laser beam source on an optical disk; variable focusing means for moving a focal position of an optical spot condensed by the objective lens; tracking means for making the optical spot track a recording track of the optical disk; spherical aberration adding means for adding a variable spherical aberration to light condensed by the objective lens; coma aberration adding means for adding a variable coma aberration to light condensed by the objective lens; a light detector for detecting reflected light from the optical disk; an operation circuit for generating a reproducing signal, a focal shift signal and a tracking shift signal from a detection signal of the light detector; servo-control means for controlling the variable focusing means, the tracking means, the spherical aberration adding means and the coma aberration adding means; and a laser drive circuit for driving the laser beam source according to a signal modulated by a record signal. The information recording/reproducing apparatus further comprises: a detected light dividing device for dividing a light flux in the vicinity of an optical axis of the reflected light into a plurality of partial light fluxes to make incident onto the light detector, and also dividing the light flux in a peripheral portion of the reflected light into a plurality of partial light fluxes to make incident onto the light detector. In this case, the operation circuit generates: a first focal shift signal and a first tracking shift signal based on a detection signal of the partial light fluxes derived from the light flux in the vicinity of the optical axis; a second focal shift signal and a second tracking shift signal based on the detection signal of the partial light fluxes derived from the light flux of the peripheral portion; a spherical aberration signal according to a difference signal between the first and second focal shift signals, and a focal shift signal according to a sum signal; and a coma aberration signal according to a difference signal between the first and second tracking shift signals, and a tracking shift signal according to a sum signal. Moreover, the servo-control means controls: the variable focusing means according to the focal shift signal; the tracking means according to the tracking shift signal; the spherical aberration adding means according to the spherical aberration signal; and the coma aberration adding means according to the coma aberration signal.
(10) In the information recording/reproducing apparatus according to (9), the detected light dividing device is divided into 8 regions by a circle, a straight line passing a center of the circle and parallel to a direction relevant to a radial direction of the optical disk, and a straight line passing a center of the circle and parallel to a direction relevant to a revolution direction of the optical disk, and the respective regions are diffraction gratings or polarizing diffraction gratings having different diffraction angles and diffraction directions from each other.
(11) In the information recording/reproducing apparatus according to (10), the light detector has a plurality of light-receiving regions in one substrate, generates a reproducing signal from a zero-order diffraction light detection signal of the detected light dividing device, and generates the focal shift signal and tracking shift signal from the +first-order diffraction light detection signal.
(12) A servo-control method for making an optical spot condensed by an objective lens track a recording track of an optical disk comprises the steps of: detecting a first focal shift signal and a first tracking shift signal by use of an inside light flux containing a light flux center of the reflected light out of the reflected light from the optical disk; detecting a second focal shift signal and a second tracking shift signal by use of an outside light flux enclosing the inside light flux out of the reflected light from the optical disk; controlling a focal position of the optical spot according to a sum signal of the first and second focal shift signals; controlling a tracking of the optical spot according to a sum signal of the first and second tracking shift signals; adding a spherical aberration to light condensed by the objective lens according to a spherical aberration signal generated based on a difference between the first and second focal shift signals; and adding a coma aberration to light condensed by the objective lens according to a coma aberration signal generated based on a difference between the first and second tracking shift signals.